This invention relates to a mixing device for mixing bone cement comprised of a dry and a liquid component. More particularly, it relates to a bone cement made from mixing powdered calcium phosphate compounds and an aqueous setting solution.
Calcium phosphate cements for use in the operating room are currently mixed in a bowl with a spatula by hand. Either sterile water or a sterile 0.25M sodium phosphate solution (0.25M Na-phos) are used as the liquid component. Such cements are described in Brown and Chow U.S. Reissue Pat. Nos. 33,161 and 33,221 and Constantz et al. U.S. Pat. No. 5,336,264, the teachings of which are incorporated herein by reference. Optimally, mixing with water results in a faster setting time of 25 to 35 minutes, while mixing the 0.25M Na-phos solution results in a setting time of 5 to 8 minutes. These solutions are mixed with the calcium phosphate powders in a liquid to powder ratio of about 0.25 to 0.30 by weight.
The mixing of powder and liquid by hand with a spatula is a technique sensitive process since each individual mixing the cement performs the operation differently. To obtain optimal mixing the powder and liquid must be pressed against the side of the bowl and the mixture continuously spread and recombined during the mixing stage. Also, the time and amount of liquid for mixing is dependent on the operators perception of a good mixing consistency and is thus very subjective. Ideally 45 to 60 seconds of hand mixing is required but the hand mixing times vary widely from individual to individual. It has been found that these mixing techniques result in a wide variation in the physical properties of the hardened cement or bone filler because largely varying amounts of energy are inputted into the material during mixing. A desire to provide alternate delivery systems (i.e. by syringe needle or cannula) also necessitate that the mixed cement flow into and from a convenient dispenser. These factors have provided the impetus for the development of automatic mixing systems.
Applicants have found that when mixing calcium phosphate cements, in particular calcium phosphate cement made from powdered tetra-calcium phosphate and di-calcium phosphate in a 1:1 molar ratio which are mixed with water or other aqueous solutions, such as those containing sodium phosphates, that a high degree of wetting of each of the powder particles is necessary for the material to harden from a paste into a cement or bone filler having optimum physical properties. Although mixing devices existed in the prior art for combining powdered and liquid components, such were not used with calcium phosphate bone cements but rather these cements were hand mixed or mixed via methods which only inputted a low amount of energy into the mixing. Such a low energy mixing system is disclosed in U.S. Pat. No. 6,083,229.
A typical prior art dental mixer is shown in FIG. 1A is used to make dental amalgams and has an almost linear motion with a travel distance of about 1.6 cm in each direction. The complete cycle (a back and forth eccentric motion) has a total travel distance of 3.2 cm. The side to side motion per cycle is about 0.5 cm. The motor runs at 3290 rpm and produces one back and forth motion each rotation or 3290 cycles per minute. However, the motor is of low power and can only mix less than one gram of material. Another mixer is made by Ivoclar which has a two pivot point eccentric motion with 2.8 cm of linear travel and 5 cm of up and down motion at one end. This system runs at a fixed 1788 rpm.
When a syringe containing a powdered calcium phosphate mineral and an aqueous solution is affixed to the prior art mixing apparatus of FIG. 1A, an intimate powder-liquid mixture results having a high degree of wetting. This mixing is superior to hand mixing. Such an improvement in wetting is shown by the ability to eject the paste through an 18 gauge or larger needle.
While this system can be modified to meet the mixing energy input and motion requirements for forming a calcium phosphate paste which can be injected, its mixing motion is not optimal and it is not a system suitable for hospital or operating room use for sterilization reasons. The mixer of the present invention can hold a large container, similar in volume to a 60 cc syringe. In its preferred embodiment, one complete cycle in this system has 3.2 cm of linear travel. There is also a maximum of 3.5 cm up and down motion on one end of the tubular syringe and a variable amount of up and down motion along the longitudinal extent of the rest of the mixing chamber.
The system of the present invention has the capability of reciprocating a container in an eccentric motion at between 800 to 2000 cycles per minute (cpm) and provides sufficient energy to form a paste which can be ejected through an 18 gauge or larger needle. The system of the present invention is a well-balanced system which minimizes vibration by means of a counter weight on the drive plate, providing a very smooth motion at high cycles per minute. This system meets the mixing energy input and motion requirements for forming a calcium phosphate paste which can be injected and may be made sterilizeable. The calcium phosphate powder may be packaged after sterilization so that sterile liquid can be added to the powder and the two components mixed together, while in a sterile package such as a syringe. Alternatively the syringe and powder combination can be sterilized together.
It is an object of this invention to provide a mixing device for mixing dry and liquid components.
It an additional object of this invention for providing a mixing device for mixing dry calcium phosphate powders and aqueous solutions which provides a high degree of coating of the dry powders with the aqueous solution.
It is yet an additional object of the invention to provide a mixing device which inputs a predetermined mixing energy into the mixture over a predetermined time in order to effect the complete wetting of the dry powders with the aqueous solution.
These and other objects are accomplished by a mixing apparatus for mixing dry and liquid components to form a paste which has a holder mounted on the apparatus. The holder is mounted for eccentric reciprocating movement of a predetermined up and down and side to side amplitude. The movement has components in at least two perpendicular directions. A drive system is operatively connected to the holder for imparting the reciprocating movement to the holder at an optimal number of cycles per minute depending on the materials to be mixed. Initially a container having a mass of dry and liquid components therein are mounted in the holder.
The mass, amplitude and cycles per minute of reciprocation are chosen to produce an energy input of at least about 3xc3x9710xe2x88x923 Joules/second. The drive system is capable of driving the holder in the reciprocating motion at least about 1600 cycles per minute. The reciprocating movement occurs in a plane with the first and second directions of movement being perpendicular along said plane. The drive system includes an input shaft rotating a drive plate with the plate having the holder pivotally mounted thereon at a first pivot point with an axis offset from an axis of the input shaft to impart an eccentric motion to the holder as said drive plate rotates about the input axis. The holder has a first end coupled to the offset first pivot point and a second end pivotally mounted on the apparatus at a second pivot point in a manner permitting a first amplitude of movement in the first direction along said plate and permitting a lower amplitude movement in the second direction along the plate.
The mixing apparatus includes a base having a circular drive plate mounted thereon with the input shaft rotating the drive plate about its central axis with the holder mounted at a first pivot point radially spaced from the central axis of the plate. The holder second pivot point is spaced along the holder from the first pivot point along the first direction. The second pivot includes a pin slidably mounted in a longitudinal slot in the base of the mixer with the slot extending in the first direction. The container is removably mounted in the holder and may be in the form of a sterile syringe pre-packaged with the dry component. The liquid component, such as a sterile aqueous solution, is brought into the syringe immediately before use by activating the plunger of a standard syringe to draw the liquid into the syringe barrel. Alternatively, both the powder and the liquid component may be added to the syringe immediately prior to mixing.
The dry component may comprise a calcium phosphate mineral powder such as a combination of tetra-calcium phosphate and di-calcium phosphate and the liquid component may be an aqueous solution of either sterile de-ionized water or a sterile water and sodium phosphate solution. The dry component also may include dry sodium phosphate particles. The liquid powder ratio in the syringe prior to mixing is generally between 0.25 and 0.3. The syringe should have an excess of empty volume of approximately 1 cc after it has been loosely filled with the powder. Addition of the liquid component will consolidate and decrease the volume of the loosely packed powder.
These and other objects and advantages of the present invention will become apparent from the following description of the accompanying drawings, which disclose several embodiments of the invention. It is to be understood that the drawings are to be used for the purposes of illustration only and not as a definition of the invention.